The pitch-based carbon fibers, i.e., the carbonized fibers obtained from a pitch according to the method of the present invention, are incomplete in crystallization and orientation of carbon hexagonal network and yet the fibers have the capability of increasing their tensile strength and modulus of elasticity greatly by a high temperature heat treatment carried out under a relaxed state whereby the growth of crystallites and orientation proceed.
The pitch-based carbon fibers of the present invention have superior workability in adaptation to various kinds of processes such as taking up on bobbins, transportation to a higher degree of carbonization or graphitization step, weaving, knitting and working for the reinforcement of resins.
The pitch-based carbon fibers of the present invention are easy to work because of their lower carbonization degree, and their cost is lower than those of higher carbonization degree. Thus even when working loss is produced, they are in advantageous state because the influence upon the cost of product is small.
The pitch-based carbon fibers according to the present invention are compliant when they are bent at a small radius of curvature, compared with carbon fibers subjected to a higher degree of carbonization and have superior characteristic properties because their bent portions undergo stress relaxation during the high temperature heat treatment applied thereafter and therefore show superior resistance to abrasion, flexion and scratching.
A method for obtaining carbon fibers by subjecting a pitch having a high softening point to melt-spinning, thermosetting the resulting spun fibers to make them infusible, followed by carbonization carried out in an inert gas atmosphere, is disclosed in Japanese official gazette of examined application (Tokuko) 15728 of 1966. This is certainly a superior production method of pitch-based carbon fibers but according to the disclosed method, it is necessary to keep thermoset pitch fibers in a stretched state during the carbonization to obtain fibers having a high modulus of elasticity. Since thermoset pitch fibers are extremely brittle, it is difficult to hold them in a stretched state. It is considered actually to be impossible to obtain high modulus carbon fibers by this method.
In order to work out a solution to this problem, a method in which an optically anisotropic pitch is used has been proposed as disclosed in Japanese official gazette of examined application (Tokuko) 8634 of 1974 and Japanese official gazette of unexamined application (Tokukai) 19127 of 1974. An optically anisotropic pitch is an easily graphitizable material and has superior properties as a raw material for high strength, high modulus carbon fibers. Particularly, there is no need for the fibers to be kept in a stretched state during the carbonization, and it is considered to be an advantageous method in view of cost and quality.
An optically anisotropic pitch can be easily made into high strength and high modulus carbon fibers, but on the other hand, the carbon fibers have such weak points that they are liable to be flawed, e.g. liable to be broken at the time of working. such weak points exist more or less in the case of brittle fibers. Glass fibers, PAN-based carbon fibers, etc. are coated by sizing agents to give lubricity and cohesiveness to fiber bundles. In the case of carbon fibers from an optically anisotropic pitch, there is a tendency to repel a sizing agent due to the harmful effect of easily graphitizable property. Since uniform coating is difficult, lack of lubricity and cohesiveness of fiber bundles are also weak points.
In order to solve these problems, Japanese unexamined patent application (Tokukai) 21911 of 1985 discloses a method in which light degree of carbonization is carried out at a temperature of 400.degree.-650.degree. C. after thermosetting. This method is effective to some extent for keeping the modulus of elasticity of the carbon fibers low and for preventing them from being flawed, but since bundles of the fibers have no cohesiveness and no lubricity, there are problems in the point of workability In order to solve such problems, it is a general method to coat the fibers with an oiling agent after carbonization but in the case of lightly carbonized pitch fibers, there is a tendency to repel an oiling agent, and there is a problem of the fibers being liable to be flawed at the time of coating with the oiling agent on one hand because strength of the fibers is not increased.
In the method for producing the pitch-based carbon fibers, the most severe condition for spin finish oils is during thermosetting which is a heat treatment carried out in an oxidative atmosphere. In order to cover the loss of the decomposed spin finish oils, it is considered advantageous to impart second oils after the thermosetting process. The problem of this method is the likelihood of creating flaws at the time of imparting the second oils because the thermoset fibers are equally or more brittle than the pitch fibers after spinning.
For imparting the second oils at this step, a spray method may be adopted, but the loss of the second oils by scattering is so great that there is an economical problem specially in the case of expensive silicone based oils.
It is an object of the present invention to overcome the brittleness, lack of lubricity and cohesiveness of bundles of carbon fibers produced from a high softening point pitch such as an optically anisotropic pitch or a pitch having characteristic carbonization properties similar to the optically anisotropic pitch.
The carbonization of pitch-based fibers is carried out generally by the heat treatment in an atmosphere of an inert gas and its effect is considered, in general, to be dependent on temperature and residence time. However, when a detailed investigation is carried out for workability, it has become clear that there is an effect due to spin finish oils remaining on the carbonized fibers. Particularly, it has become clear that the effect for lubricity and cohesiveness of fiber bundles are notable. Further there seems to be a difference of effectiveness between apparatus for carbonization.
Though the reason is not clear, it is inferred that a shape of fiber bundles, which is formed at the stage of bundles of pitch fibers possessing a good lubricity and cohesiveness, is maintained by spin finish oils remaining on the carbonized fibers in a slight amount and that this gives a large influence upon workability.